Printing according to the "ink jet printing" principle, and apparatus for carrying out such printing operations, are well known. In general terms, a fluid ink is forced, under pressure and often at an elevated temperature, through a very small orifice in a printing head.
In so-called "continuous" jet printing operations, ink droplets are passed through a charging area wherein individual droplets receive an electrical charge in response to a signal. The droplets then pass through an electrical field, causing a varied deflection of the individual droplets dependent on the intensity of the charge and field. The droplets are produced continuously, and are directed to the substrate to be printed or, alternatively, to a by-pass gutter.
Due to the nature of the "continuous" jet printing process, inks used therein must be conductive so as to accept a charge. Such inks are generally water based, as aqueous solutions or dispersions are easily rendered conductive. Water based inks are generally characterized as having a low viscosity (which tends to vary with temperature) and high volatility. Both characteristics can be disadvantageous.
If the viscosity of a jet printing ink is excessively low, misting and the production of stray droplets can result, which in turn result in poor print quality. High volatility of a jet printing ink can result in clogging of jet nozzles due to evaporation.
Water-based inks are capable of use on plain paper (i.e. a paper not bearing a pigment coating) and are capable of providing a high dye concentration on the surface of the paper so that it is possible to produce fine sized droplets and to avoid build-up of dyestuff on the substrate surface while providing high color intensity.
So-called "drop on demand" (DOD) systems differ from continuous jet printing systems in that ink droplets are expelled from the nozzle of a printing head only when required during the printing process.
In a DOD printing system, ink is provided from a reservoir via a supply system, to a nozzle/actuator system in the printing head. The actuators are generally of the piezoelectric or bubble type, and pressurize ink upon activation to force this ink from a chamber defined by the actuator through an associated jet nozzle associated therewith.
Since inks used in DOD Systems need not be conductive, and preferably are non-conductive, they have generally been formulated using primarily non-aqueous solvents such as ethylene glycols, particularly diethylene glycols, which are characterized as having low volatility. As such, ethylene glycol and similar solvent based inks do not evaporate quickly and, thus, exhibit reduced clogging problems. By a non-conductive ink is meant an ink which is substantially free of ionisable material.
However, ethylene glycol and other non-aqueous solvents have a relatively high viscosity (which varies significantly with temperature) and thus require relatively more energy for production of droplets.
Solvent based inks exhibit desirable drying characteristics and are biologically stable while water based inks generally tend to sorb air from the environment and are susceptible to biological fouling.
Another class of inks used in jet printing are referred to in the art as "hot melt" inks and comprise wax or other thermoplastic materials such that the ink is solid or semisolid at ambient temperatures but fluid at elevated temperatures. The heated ink fluid solidifies when it comes in contact with a paper target.
Such inks generally tend to exhibit excellent dye stability, lightfastness, and low solvent volatility. However, these inks accommodate only relatively low dye concentration and therefore necessitate the production of relatively large drops in order to provide acceptable color intensity. Also, the thermoplastic component of such inks tends to form raised spots which are easily smeared by rubbing.
Hot melt jet inks tend to be highly viscous and thus require high energy input in order to eject the drop from the printer nozzle. Also, since the entire ink must be rendered fluid by heating, high energy inputs and lengthy time periods are required to reach the printer operating temperature.